Dental Materials With Light-Induced Reversible Coloring

ABSTRACT

The invention relates to a radically polymerizable dental material having photochromic properties, which contains at least one compound of formula (I) and optionally radically polymerizable monomers, an initiator for radical polymerization and other components.

The present invention relates to heat- and/or light-curing compositionswith light-induced reversible colouring, such as polymerization resinsand composites, which are suitable in particular as dental fillingcomposites, veneering materials, dental adhesives and dental coatingmaterials, as well as for the production of inlays and onlays.

In the case of plastics, colourants are added to the polymer asadditives to set the desired colour and transparency. A distinction ismade between dyes and pigments. Dyes have an organic nature and areusually soluble in organic solvents, while pigments are solid particleswith particle sizes between approx. 0.01 and approx. 1 μm. Pigments aredivided into organic and inorganic pigments (cf. Taschenbuch derKunststoff-Additive, Ed. R. Gächter, H. Müller, 3^(rd) edition, CarlHanser Verlag, Munich and Vienna 1989, 663-736). In the case of dentalmaterials used in dentistry and in particular in the case of highlyaesthetic filling composites mixtures of different inorganic pigmentsare used to set the colour, which are characterized by low solubility inorganic solvents and fats as well as by a very good colour stability. Inaddition to the permanent colouration of dental materials, it isadvantageous in some situations to be able to make the dental materialmore visible temporarily by reversible colouring. Examples of this are areversible colouring for the identification of thin layers, such as inthe case of tooth surfaces treated with adhesive or cement surpluses, aswell as for making fissure sealants visible.

EP 0 744 172 A1 discloses photochromic dental materials which contain aphotochromic material such as for example a photochromic dye, aphotochromic glass, a photochromic ceramic and/or a photochromic glassceramic. By brief irradiation with light the photochromic dentalmaterial can be converted to a coloured state which makes a betterdifferentiation from the natural tooth structure possible. Adisadvantage is that the subsequent decolouration sometimes extends overseveral hours, which can impair the light-induced material hardening. Inaddition, the dentist cannot control the ultimate colouring of therestoration in one session.

A distinction is made between photochromic substances of the T-type, inwhich the reverse reaction proceeds predominantly thermally after theirradiation has ended, and substances of the P-type, in which thereverse reaction proceeds predominantly photochemically, i.e. istriggered by light of a different wavelength (cf. S. N. Corns, A. M.Partingtom, A. D. Towns, Color. Technol. 125 (2009) 249-261). Examplesof photochromic substances of the P-type are diarylethenes withheterocyclic aryl groups (cf. M. Irie, Chem. Rev. 100 (2000) 1685-1716).The use of photochromic diarylethenes in dental materials is not knownaccording to the state of the art.

The object of the invention is to provide materials for dentalapplications, the colour of which can be reversibly altered byirradiation with light and which do not have the disadvantagesassociated with the state of the art.

The object is achieved according to the invention by radicallypolymerizable materials which contain at least one photochromicdiarylethene according to M. Irie, Chem. Rev. 100 (2000) 1685-1716).

Radically polymerizable dental materials which contain a compoundaccording to general formula I are preferred:

in which the variables have the following meanings:

-   X, Y independently of each other in each case are O, S, Se, CH₂,    N—R¹¹ or CR¹²R¹³, wherein R¹¹ to R¹³ independently of each other in    each case are a branched or unbranched C₁-C₁₆ alkyl radical, aryl or    —CH₂-aryl, wherein X and Y preferably have the same meaning;-   R¹, R² independently of each other in each case are a branched or    unbranched C₁-C₃ alkyl radical, wherein these alkyl radicals can be    substituted by one or more fluorine atoms;-   R⁵; R⁶ independently of each other in each case are halogen, CN,    COOH, COOR⁴, CH₂OH, CO—NH₂, CH₂—NH₂, wherein R⁴ is a C₁-C₆ alkyl    radical, aryl or alkyl-aryl, or R^(y) and R⁶ together form a    —(CH₂)_(n)—, —C(═O)—O—C(═O)— or —C(═O)—NR³—C(═O)— group, wherein n    is 3 or 4 and R³ is H, a C₁-C₆ alkyl radical, aryl or alkyl-aryl and    wherein in the —(CH₂)_(n)— group one or more, preferably all H atoms    can be replaced by F;-   R⁷⁻¹⁰ independently of each other in each case are H, a C₁-C₁₂ alkyl    radical, which can be interrupted by O, —O—C(═O)—NH— or phenylene,    or an aromatic C₆-C₁₀ hydrocarbon radical, wherein these radicals in    each case can carry a radically polymerizable group, or R⁷ and R⁸    and/or R⁹ and R¹⁰, together with the C atoms to which they are    bonded, form a benzene ring, which can be unsubstituted or which can    carry 1 to 4 substituents, which are selected from halogen, CN,    —CO-aryl, —CO—CH₂-aryl, —CO—O-aryl, —CO—O—CH₂-aryl, branched or    unbranched C₁-C₁₆ alkyl radicals, —O— alkyl, —CO-alkyl and    —CO—O-alkyl, wherein alkyl in each case stands for a branched or    unbranched C₁-C₁₆ alkyl radical and wherein all alkyl radicals can    be substituted by one or more fluorine atoms;    wherein according to the invention those compounds which contain at    least one radically polymerizable group are preferred.

Formula (I) and the other formulae shown herein cover all stereoisomericforms as well as mixtures of different stereoisomeric forms, such ase.g. racemates. The formulae cover only those compounds that arecompatible with the chemical valence theory. The indication that aradical can be interrupted by a hetero atom such as O is to beunderstood to mean that the O atoms are inserted into the carbon chainor the carbon ring of the radical, i.e. are bordered on both sides bycarbon atoms. The number of hetero atoms is therefore at least 1 lessthan the number of carbon atoms, and the hetero atoms cannot beterminal. In the case of hydrocarbon radicals which contain carbon andhetero atoms, the number of hetero atoms is always less than the numberof carbon atoms regardless of substituents. C₁ radicals cannot beinterrupted.

The term aryl herein stands in each case for an aromatic hydrocarbonradical, preferably a phenyl radical, which can be substituted andpreferably is unsubstituted.

Preferred polymerizable groups which can be present as substituents inthe radicals R are vinyl, styryl, acrylate (CH₂═CH—CO—O—), methacrylate(CH₂═C(CH₃)—CO—O—), acrylamide (CH₂═CH—CO—NR¹⁴— where R¹⁴═H or C₁-C₈alkyl), methacrylamide (CH₂═C(CH₃)—CO—NH—), particularly preferably(meth)acrylate, methacrylamide and/or N-alkylacrylamide.

Compounds according to Formula I in which the variables have thefollowing meanings are preferred:

-   X, Y independently of each other are O, S or N—R¹¹, wherein R¹¹ is a    branched or unbranched C₁-C₆ alkyl radical;-   R¹, R² independently of each other in each case are a C₁-C₂ alkyl    radical, wherein these alkyl radicals can be substituted by one or    more fluorine atoms;-   R⁵; R⁶ together form a —(CH₂)_(n)— group, wherein n is 3 or 4 and    wherein in the —(CH₂)_(n)— group one or more, preferably all H atoms    can be replaced by F;-   R⁷⁻¹⁰ independently of each other in each case are H or a C₁-C₆    alkyl radical, which can be interrupted by O or —O—C(═O)—NH—,    wherein one or more H atoms can be substituted by fluorine atoms and    wherein 1 or 2 of the radicals R⁷⁻¹⁰ carry a radically polymerizable    group, preferably a (meth)acrylate group.

Compounds according to Formula I in which the variables have thefollowing meanings are particularly preferred:

-   X, Y in each case are S;-   R¹, R² in each case are methyl;-   R⁵; R⁶ together form a —(CH₂)_(n)— group, wherein n is 3 and wherein    preferably all H atoms are replaced by F;-   R⁷, R⁹ independently of each other in each case are H or a C₁-C₃    alkyl radical;-   R⁸, R¹⁰ independently of each other in each case are a C₁-C₆ alkyl    radical, which can be interrupted by O or —O—C(═O)—NH— and which    carries a terminal radically polymerizable group, preferably a    methacrylate group.

Furthermore, those compounds of Formula I in which R⁷ and R⁸ stand for agroup with Formula Ia and R⁹ and R¹⁰ stand for a group with Formula Ibare preferred:

Compounds of this type can be represented by the following Formula (II):

R^(a), R^(b), R^(c), R^(d) and R^(a′), R^(b′), R^(c′), R^(d′)independently of each other in each case mean:

-   -   H, halogen, CN, a branched or unbranched C₁-C₁₆ alkyl radical,        —O-alkyl, —CO-alkyl, —CO—O-alkyl, wherein alkyl in each case        stands for a branched or unbranched C₁-C₁₆ alkyl radical and        wherein all alkyl radicals can be substituted by one or more        fluorine atoms, —CO-aryl, —CO—CH₂-aryl, —CO—O-aryl,        —CO—O—CH₂-aryl;    -   wherein the radicals R^(a), R^(b), R^(c), R^(d) or R^(3′),        R^(b′), R^(c′), R^(d′) can be linked to each other, forming one        or more non-aromatic or preferably aromatic rings and in        particular annulated aromatic ring systems, which preferably        have 2 to 5 rings, wherein the rings or ring systems can be        substituted or preferably unsubstituted.

The remaining variables have the above-specified meanings.

A preferred group of compounds of Formula II are those compounds inwhich the variables have the following meanings, which can be chosenindependently of each other:

-   X, Y independently of each other in each case are 0, N—R¹¹, wherein    R¹¹ is a branched or unbranched C₁-C₁₆ alkyl radical, N-aryl or    N—CH₂-aryl;-   R¹, R² independently of each other in each case are a branched or    unbranched C₁-C₆ alkyl radical, wherein the alkyl radicals can be    substituted by one or more fluorine atoms;-   R⁵, R⁶ independently of each other in each case are halogen, CN,    COOH, COOR⁴, wherein R⁴ is H, alkyl, aryl, alkyl-aryl, CH₂OH, CO—NH₂    or CH₂—NH₂, or R⁵ and R⁶ together form a —C(═O)—O—C(═O)—,    —C(═O)—NR³—C(═O)— or —(CH₂)_(n)— group, wherein n is 3 or 4 and one    or more, preferably all H atoms can be replaced by F and wherein R³    is H or a C₁-C₆ alkyl radical, aryl or alkyl-aryl;    R^(a), R^(b), R^(c), R^(d) and R^(a′), R^(b′), R^(c′), R^(d′)    independently of each other are    -   H, halogen, a branched or unbranched C₁-C₁₆ alkyl radical,        —O-alkyl, —CO—O-alkyl, wherein alkyl in each case stands for a        branched or unbranched C₁-C₁₆ alkyl radical and wherein all        alkyl radicals can be substituted by one or more fluorine atoms,        —CO-aryl, —CO—CH₂-aryl, —CO—O-aryl, —CO—O—CH₂-aryl;    -   wherein the radicals R^(a), R^(b), R^(c), R^(d) and/or the        radicals R^(a′), R^(b′), R^(c′), R^(d′) can be linked to each        other, forming one or more aromatic rings, preferably an        annulated aromatic ring system with 2 to 3 rings, wherein the        rings or ring systems can be substituted and preferably are        unsubstituted.

Compounds of Formula II in which the variables have the followingmeanings, which can be chosen independently of each other, are furtherpreferred:

-   X, Y independently of each other in each case are 0, N—R¹¹, wherein    R¹¹ is a branched or unbranched C₁-C₁₆ alkyl radical;-   R¹, R² independently of each other in each case are a methyl group,    which can be substituted by one or more fluorine atoms;-   R⁵, R⁶ independently of each other in each case are COOH, CH₂OH,    CO—NH_, CH₂—NH₂ or R⁵ and R⁶ together form a —C(═O)—O—C(═O)— or    —(CH₂)_(n)— group, wherein n is 3 or 4 and one or more, preferably    all H atoms can be replaced by F, particularly preferably R⁵ and R⁶    together form a —(CF₂)₃— group;    R^(a), R^(b), R^(c), R^(d) and R^(a′), R^(b′), R^(c′), R^(d′)    independently of each other are    -   H, halogen, a branched or unbranched C₁-C₁₆ alkyl radical,        —O-alkyl, wherein alkyl stands for a branched or unbranched        C₁-C₁₆ alkyl radical and wherein all alkyl radicals can be        substituted by one or more fluorine atoms,    -   wherein the radicals R^(a), R^(b), R^(c), R^(d) and/or the        radicals R^(a′), R^(b′), R^(c′), R^(d′) can be linked to each        other, forming one or more aromatic rings, which are preferably        not substituted.

Compounds of Formula II are also called diarylethenes in the following.

Preferred compounds of Formulae I and II are:

In all cases, compounds of Formulae I and II which carry one or moreradically polymerizable groups, in particular vinyl, (meth)acryl and/or(meth)acrylamide groups are particularly preferred. Such compounds arecovalently incorporated into the organic polymer matrix during theradical hardening of the dental materials and then can no longer bewashed out, with the result that the photochromic properties are alsopreserved over longer periods of time. Furthermore, the ability tomigrate is substantially reduced in this way.

The compounds of Formulae I and II used according to the invention arecharacterized by a reversible light-induced colouring at differentwavelengths, i.e. they can be coloured by brief irradiation with lightof the wavelength λ₁ and decoloured again by irradiation with light ofthe wavelength λ₂, wherein the decolouration is preferably effected withlonger-wave visible light. This procedure surprisingly proceeds highlyefficiently, even when the compounds of Formula I or II are embedded inamorphous polymer networks, so-called polymer glasses, bycopolymerization. According to the invention, compounds of Formulae Iand II in which λ₂>λ₁+50 nm are preferred. λ₁ preferably lies in theultraviolet range and λ₂ in the visible spectral range. Here, λ₁ is tobe chosen such that, during irradiation, the photoinitiator contained inthe material and the compound I do not affect each other substantiallyin terms of functionality. The incorporation of I into the dentalvarnish does not result in a visible discolouration of the seal.

The compounds of Formulae I and II are P-type chromophores. Dentalmaterials which contain a compound of Formulae I and II can bedecoloured in a targeted manner by irradiation with light of thewavelength λ2. This makes a high degree of flexibility in the processingof the materials possible, because the processing time can be chosen asdesired, unlike with T-type chromophores.

The dental materials according to the invention based on photochromicadditives of general formulae I and II can thus be coloured or madevisible in a targeted manner by brief irradiation with light of thewavelength λ₁. The coloured dental materials can then be decolouredagain by brief irradiation, in the range of seconds, with light of thewavelength λ₂. The reactions proceeding here are shown by way of examplefor Formula II:

The uncoloured compounds are characterized by an open ring (left-handformula), the coloured compounds by a closed ring (right-hand formula).The materials according to the invention are preferably sold in theuncoloured form, and the compounds of Formulae I and II are accordinglydefined here using the formulae for the uncoloured compounds. However,it goes without saying that those materials which contain thecorresponding coloured compounds are also a subject of the invention.

The colour effect which can be achieved depends mainly on theconcentration of photochromic compound of Formula I or II, the degree oftransformation, the rate of transformation and the extinctioncoefficient of the photochemically produced compound. Compounds whichhave a quantum yield of more than 10% are preferred. The extinctioncoefficient preferably lies above 10,000 M⁻¹ cm⁻¹.

According to the invention, those compounds of Formulae I and II arepreferred which can be excited with a wavelength λ₁ smaller than 400 nm,in particular with UV light in the range of 320-395 nm, wherein thosecompounds which have no or only a slight intrinsic colour areparticularly preferred. The compounds of Formulae F1-F11 meet theserequirements.

Commercially available photochromic compounds with a diarylethenestructure (according to Formulae I and II) are:

The dental materials according to the invention contain, in addition tothe monomers of general formula I, preferably additionally furtherradically polymerizable monomers, in particular mono- and/orpolyfunctional (meth)acrylic acid derivatives. Materials which containat least one multifunctional (meth)acrylate or a mixture of mono- andmultifunctional (meth)acrylates as radically polymerizable monomer areparticularly preferred. By monofunctional (meth)acrylates is meantcompounds with one, by polyfunctional (meth)acrylates is meant compoundswith two or more, preferably 2 to 4, radically polymerizable groups.According to a quite particularly preferred embodiment, the compositionsaccording to the invention contain at least one dimethacrylate or amixture of mono- and dimethacrylates. Materials which contain mono- andmultifunctional (meth)acrylates as radically polymerizable monomer areparticularly suitable as dental materials. In all cases, methacrylatesare preferred as comonomers. It was found that the monomers of Formula Iand in particular the preferred compounds of Formula I with thecomonomers named here have good compatibility and form homogeneousmixtures which, in the case of polymerization, produce materials withvery good mechanical properties.

Preferred mono- or polyfunctional methacrylates are methyl, ethyl,hydroxyethyl, butyl, benzyl, tetrahydrofurfuryl orisobornyl(meth)acrylate, p-cumylphenoxyethylene glycol methacrylate(CMP-1E), bisphenol A di(meth)acrylate, bis-GMA (an addition product ofmethacrylic acid and bisphenol A diglycidylether), UDMA (an additionproduct of 2-hydroxyethyl methacrylate (HEMA) and2,2,4-trimethylhexamethylene diisocyanate), TMX-UDMA (an additionproduct of a mixture of HEMA and hydroxypropyl methacrylate (HPMA) withα,α,α′,α′-tetramethyl-m-xylylene diisocyanate (TMXDI)),bis(meth-acryloyloxymethyl)tricyclo[5.2.1.]decane (TCDMA), ethoxylatedor propoxylated bisphenol A di(meth)acrylate, such as e.g. the bisphenolA dimethacrylate2-[4-(3-methacryloyloxyethoxy-ethyl)phenyl]-2-[4-(3-methacryloyloxyethyl)phenyl]-propane)(SR-348c) with 3 ethoxy groups or2,2-bis[4-(2-(meth)acryl-oxypropoxy)phenyl]propane, di-, tri- ortetraethylene glycol di(meth)acrylate, trimethylolpropanetri(meth)acrylate, pentaerythritol tetra(meth)acrylate, as well asglycerol di(meth)acrylate, 1,4-butanediol di(meth)acrylate,1,10-decanediol di(meth)acrylate or 1,12-dodecanediol di(meth)-acrylateor glycerol trimethacrylate (GTMA).

N-mono- or -disubstituted acrylamides, such as e.g. N-ethyl acrylamide,N,N-dimethyl acrylamide, N-(2-hydroxyethyl)acrylamide orN-methyl-N-(2-hydroxyethyl)acrylamide, or N-monosubstitutedmethacrylamides, such as e.g. N-ethylmethacrylamide orN-(2-hydroxyethyl)ethacrylamide as well as N-vinylpyrrolidone arefurther preferred. These monomers are characterized by a low viscosityand a high hydrolytic stability and are particularly suitable asdiluting monomers.

Crosslinking pyrrolidones, such as e.g.1,6-bis(3-vinyl-2-pyrrolidonyl)-hexane or commercially availablebisacrylamides, such as methylene- or ethylenebisacrylamide, orbis(meth)acrylamides, such as e.g.N,N′-diethyl-1,3-bis(acrylamido)-propane,1,3-bis(methacrylamido)-propane, 1,4-bis(acrylamido)-butane or1,4-bis(acryloyl)piperazine, which can be synthesized by reaction fromthe corresponding diamines with (meth)acrylic acid chloride, are alsopreferred. These monomers are characterized by a high hydrolyticstability and are particularly suitable as crosslinking monomers.

Alternatively or in addition, the dental materials according to theinvention can contain, in addition to the comonomers named above, one ormore acid-group-containing radically polymerizable monomers (adhesivemonomers) as additional monomers. These give the materials self-adhesiveand/or self-etching properties.

The compounds of Formulae I and II are stable under acidic conditionsand can be converted to the coloured ring-closed form with UV light. Thereverse reaction with visible light likewise proceeds reversibly.

Preferred acid-group-containing monomers are polymerizable carboxylicacids, phosphonic acids, phosphoric acid esters and sulphonic acids.

Preferred carboxylic acids are maleic acid, acrylic acid, methacrylicacid, 2-(hydroxymethyl)acrylic acid,4-(meth)-acryloyloxyethyltrimellitic acid,10-methacryloyloxydecylmalonic acid,N-(2-hydroxy-3-methacryloyloxypropyl)-N-phenyl-glycine and4-vinylbenzoic acid.

Preferred phosphonic acid monomers are vinyl phosphonic acid,4-vinylphenyl phosphonic acid, 4-vinylbenzyl phosphonic acid,2-methacryloyloxyethyl phosphonic acid, 2-methacrylamidoethyl phosphonicacid, 4-methacrylamido-4-methylpentyl phosphonic acid,2-[4-(dihydroxyphosphoryl)-2-oxa-butyl]-acrylic acid or2-[4-(dihydroxyphosphoryl)-2-oxa-butyl]-acrylic acid ethyl and-2,4,6-trimethyl phenyl ester.

Preferred acidic polymerizable phosphoric acid esters are2-methacryloyloxypropyl mono- or dihydrogen phosphate,2-methacryloyloxyethyl mono- or dihydrogen phosphate,2-methacryl-oyloxyethylphenyl hydrogen phosphate, dipentaerythritolpenta-methacryloyloxy phosphate, 10-methacryloyloxydecyl dihydrogenphosphate, phosphoric acid mono-(1-acryloyl-piperidin-4-yl) ester,6-(methacrylamido)hexyl dihydrogen phosphate and1,3-bis-(N-acryloyl-N-propyl-amino)-propan-2-yl dihydrogen phosphate.

Preferred polymerizable sulphonic acids are vinyl sulphonic acid,4-vinylphenyl sulphonic acid or 3-(methacryl-amido)propylsulphonic acid.

Particularly preferred acid monomers are4-(meth)acryloyloxyethyltrimellitic anhydride,10-methacryloyloxydecylmalonic acid, 2-methacryloyloxyethyl phosphonicacid, 2-methacrylamidoethyl phosphonic acid,2-[4-(dihydroxyphosphoryl)-2-oxa-butyl]acrylic acid,2-[4-(dihydroxyphosphoryl)-2-oxa-butyl]-acrylic acid ethyl or-2,4,6-trimethyl phenyl ester, 2-methacryloyloxypropyl mono- ordihydrogen phosphate, 2-methacryloyloxyethyl mono- or dihydrogenphosphate, 2-meth-acryloyloxyethylphenyl hydrogen phosphate,10-methacryloyloxydecyl dihydrogen phosphate, 6-(methacrylamido)hexyldihydrogen phosphate and 1,3-bis-(N-acryloyl-N-propyl-amino)-propan-2-yldihydrogen phosphate.

The dental materials according to the invention preferably also containan initiator for the radical polymerization.

Benzophenone, benzoin and derivatives thereof or α-diketones orderivatives thereof, such as 9,10-phenanthrenequinone,1-phenyl-propane-1,2-dione, diacetyl or 4,4′-dichlorobenzil arepreferred for the initiation of the radical photopolymerization.Camphorquinone and 2,2-dimethoxy-2-phenyl-acetophenone are particularlypreferably used and α-diketones in combination with amines as reducingagent, such as e.g. 4-(dimethylamino)benzoic acid ethyl ester,N,N-dimethylamino-ethyl methacrylate, N,N-dimethyl-sym.-xylidine ortriethanol-amine are particularly preferably used. Norrish type Iphotoinitiators, above all acyl- or bisacylphosphine oxides,monoacyltrialkyl or diacyldialkyl germanium compounds, such as e.g.benzoyltrimethylgermanium, dibenzoyldiethylgermanium orbis(4-methoxybenzoyl)diethylgermanium are also particularly suitable.Advantageously, mixtures of the different photoinitiators can also beused, such as e.g. dibenzoyldiethylgermanium in combination withcamphorquinone and 4-dimethyl-aminobenzoic acid ethyl ester.

Preferably, redox-initiator combinations, such as e.g. combinations ofbenzoyl peroxide with N,N-dimethyl-sym.-xylidine orN,N-dimethyl-p-toluidine, are used as initiators for a polymerizationcarried out at room temperature. In addition, redox systems consistingof peroxides or hydroperoxides and such reducing agents, as e.g.ascorbic acid, barbiturates, thioureas or sulphinic acids, are alsoparticularly suitable.

Furthermore, the dental materials according to the invention preferablyalso contain at least one organic or particularly preferably inorganicparticulate filler. Fillers based on oxides such as SiO₂, ZrO₂ and TiO₂or mixed oxides of SiO₂, ZrO₂, ZnO and/or TiO₂, nanoparticulate ormicrofine fillers such as pyrogenic silicic acid or precipitated silicicacid (weight-average particle size of 10-1000 nm) as well as minifillers such as quartz, glass ceramic or X-ray opaque glass powders ofe.g. barium or strontium aluminium silicate glasses (weight-averageparticle size of 0.2-10 μm) are preferred. Further preferred fillers areX-ray opaque fillers, such as ytterbium trifluoride or nanoparticulatetantalum(V) oxide or barium sulphate or mixed oxides of SiO₂ withytterbium(III) oxide or tantalum(V) oxide (weight-average particle sizeof 10-1000 nm).

To improve the bond between the filler particles and the crosslinkedpolymerization matrix, SiO₂-based fillers can be surface-modified withmethacrylate-functionalized silanes, such as e.g.3-methacryloyloxypropyltrimethoxysilane. For the surface-modification ofnon-silicate fillers, e.g. of ZrO₂ or TiO₂, functionalized acidicphosphates, such as e.g. 10-methacryloyloxy dihydrogen phosphate canalso be used.

Depending on the desired intended use, the dental materials according tothe invention can preferably also contain a solvent, in particularwater, ethanol or a mixture thereof. Optionally, the compositions usedaccording to the invention can also contain further additives, above allstabilizers, such as e.g. polymerization stabilizers, flavourings,colourants, microbiocidal active ingredients, fluoride-ion-releasingadditives, optical brighteners, fluorescent agents, plasticizers and/orUV absorbers.

According to the invention, those dental materials which contain thefollowing components are preferred:

-   a) 0.0001 to 5.0 wt.-%, preferably 0.001 to 3.0 wt.-% and    particularly preferably 0.01 to 1.0 wt.-% of at least one compound    of general formula I,-   b) 0.01 to 10 wt.-%, particularly preferably 0.1 to 3.0 wt.-% of at    least one initiator, and optionally-   c) 0 to 80 wt.-%, preferably 0 to 60 wt.-% and particularly    preferably 5 to 50 wt.-% other monomer(s), and optionally-   d) 0 to 80 wt.-% filler(s), and optionally-   e) 0 to 70 wt.-% solvent.

Dental materials for use as cement or filling composite preferably havethe following composition:

-   a) 0.0001 to 5.0 wt.-%, preferably 0.001 to 3.0 wt.-% and    particularly preferably 0.01 to 1.0 wt.-% of at least one compound    of general formula I,-   b) 0.01 to 10 wt.-%, particularly preferably 0.1 to 3.0 wt.-% of at    least one initiator,-   c) 0 to 50 wt.-%, preferably 0 to 40 wt.-% and particularly    preferably 5 to 40 wt.-% other monomer(s),-   d) 10 to 80 wt.-%, preferably 20 to 80 wt.-%, particularly    preferably 30 to 80 wt.-% filler(s).

Dental materials for use as adhesives or coating material preferablyhave the following composition:

-   a) 0.0001 to 5.0 wt.-%, preferably 0.001 to 3.0 wt.-% and    particularly preferably 0.01 to 1.0 wt.-% of at least one compound    of general formula I,-   b) 0.01 to 10 wt.-%, particularly preferably 0.1 to 3.0 wt.-% of at    least one initiator,-   c) 0 to 80 wt.-%, preferably 5 to 60 wt.-% and particularly    preferably 5 to 50 wt.-% other monomer(s),-   d) 0 to 20 wt.-% filler(s),-   e) 0 to 70 wt.-%, preferably 0 to 60 wt.-% and particularly    preferably 0 to 50 wt.-% solvents, in particular water and/or    ethanol.

Dental materials for the production of prostheses or artificial teethpreferably have the following composition:

-   a) 0.0001 to 5.0 wt.-%, preferably 0.001 to 3.0 wt.-% and    particularly preferably 0.01 to 1.0 wt.-% of at least one compound    of general formula I,-   b) 0.01 to 10 wt.-%, particularly preferably 0.1 to 3.0 wt.-% of at    least one initiator, and-   c) 0 to 80 wt.-%, preferably 0 to 60 wt.-% and particularly    preferably 5 to 50 wt.-% other monomer(s), and-   d) 0 to 40 wt.-% filler(s).

Dental materials for the production of inlays, onlays, crowns or bridgespreferably have the following composition:

-   a) 0.0001 to 5.0 wt.-%, preferably 0.001 to 3.0 wt.-% and    particularly preferably 0.01 to 1.0 wt.-% of at least one compound    of general formula I,-   b) 0.01 to 10 wt.-%, particularly preferably 0.1 to 3.0 wt.-% of at    least one initiator, and optionally-   c) 0 to 60 wt.-%, preferably 0 to 50 wt.-% and particularly    preferably 5 to 50 wt.-% other monomer(s), and optionally-   d) 10 to 80 wt.-%, preferably 15 to 80 wt.-% and particularly    preferably 20 to 80 wt.-% filler(s).

Unless otherwise stated, all quantities relate to the total mass of thematerials. The individual quantity ranges can be chosen separately.

Those materials which consist of the named components are particularlypreferred. Furthermore, those materials in which the individualcomponents are in each case selected from the above-named preferred andparticularly preferred substances are preferred.

The materials according to the invention are particularly suitable asdental materials, in particular as dental cements, filling compositesand veneering materials and as materials for the production ofprostheses, artificial teeth, inlays, onlays, crowns and bridges. Theyare characterized by reversible photochromic properties.

The dental materials are suitable primarily for intraoral application bythe dentist to restore damaged teeth (clinical materials), i.e. fortherapeutic use, e.g. as dental cements, filling composites andveneering materials. However, they can also be used extraorally, forexample in the production or repair of dental restorations, such asprostheses, artificial teeth, inlays, onlays, crowns and bridges(technical materials).

The invention is explained in more detail below by means of embodimentexamples.

EMBODIMENT EXAMPLES Example 1

Methacrylate Resin and Fissure Sealant Based on a Photochromic Additive

The following compositions (proportions in wt.-%) were prepared in amixing bowl (resin) or with a three roll mill (Table 1):

TABLE 1 Composition of the materials [wt. - %] Component Resin Fissuresealant Bis-GMA 19.90 11.98 UDMA 39.44 23.74 TEGDMA¹⁾ 39.66 23.54 CQ²⁾0.10 0.06 EDMAB³⁾ 0.40 0.24 B2287⁴⁾ 0.50 0.50 Aerosil Ox-50⁵⁾ — 15.64HDK 2000⁶⁾ — 4.00 Glass filler⁷⁾ — 20.30 ¹⁾Triethylene glycoldimethacrylate ²⁾Camphorquinone ³⁾p-Dimethylamino-benzoic acid ethylester ⁴⁾Photochromic additive:1,2-bis[2-methylbenzo-[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-1-cyclopentene (TCI Europe):

⁵⁾Silanized pyrogenic silicic acid (Degussa) with a specific surfacearea (BET) of approx. 50 m²/g ⁶⁾Pyrogenic silicic acid (Wacker) with aspecific surface area (BET) of approx. 200 m²/g ⁷⁾Silanized glassionomer filler G018-090 (Schott) with a particle diameter d₅₀ of 3 μm

Round test pieces (diameter 10 mm, height: 1 mm) were prepared from thecompositions and hardened for 2×1 minute with a dental light source(Spectramat®, Ivoclar Vivadent AG). Colourless (resin) or white (fissuresealant) test pieces which discoloured to pink when irradiated (2 s)with an LED (λ₁=385 nm) resulted. It was possible to decolour the testpieces again rapidly by irradiation (2 s) with an LED (λ₂=470 nm). Thealternate discolouration and decolouration were able to be repeated atleast 20× without restrictions.

Example 2 Synthesis of the Polymerizable Photochromic Compound3,3,4,4,5,5-hexafluoro-1,2-bis(5-methacryloyloxymethyl-2-methyl-3-thienyl)cyclopent-1-ene1^(st) Stage: 4-Bromo-5-methylthiophene-2-carboxaldehyde

A solution of bromine (19.18 g; 0.12 mol) in acetic acid (50 ml) wasadded dropwise to a solution of 5-methyl-2-thiophenecarboxaldehyde(12.62 g; 97.9 mmol) in acetic acid (80 ml) under exclusion of light.The temperature was kept below 30° C. by cooling in a water bath. Thereaction mixture was stirred for 72 h at RT and then carefully pouredinto saturated aqueous Na₂CO₃ solution (500 ml). After the gas evolutionhad abated, diethyl ether (400 ml) was added and the phases wereseparated. The aqueous phase was extracted with diethyl ether (2×100ml). The combined organic phases were washed with saturated aqueousNaHCO₃ solution (150 ml), dried over Na₂SO₄, filtered and concentratedon a rotary evaporator. The crude product was purified by means ofcolumn chromatography (SiO₂, n-hexane/ethyl acetate 9:1; R_(f)=0.47).14.67 g (71.5 mmol; 73%) of a yellowish solid was obtained.

¹H-NMR (CDCl₃, 400 MHz): δ=2.48 (s, 3H; CH₃), 7.59 (s, 1H, ═CH), 9.77(s, 1H, CHO).

¹³C-NMR (CDCl₃, 100.6 MHz): δ=15.9 (CH₃), 111.2 (═C), 138.7 (═CH), 140.1(═C), 145.8 (═C), 181.6 (C═O).

2^(nd) Stage: 4-Bromo-2-hydroxymethyl-5-methylthiophene

Sodium borohydride (5.36 g; 0.142 mol) was added to a solution of4-bromo-5-methylthiophene-2-carboxaldehyde (24.20 g; 0.118 mol) inethanol (250 ml) at 0° C. The reaction mixture was stirred for 1 h at 0°C. and for 2 h at ambient temperature. Then saturated aqueous NH₄Clsolution (100 ml), water (200 ml) and ethyl acetate (300 ml) were addedand the phases were separated. The aqueous phase was extracted withethyl acetate (100 ml). The combined organic phases were washed withsaturated aqueous NaHCO₃ solution (100 ml) and saturated aqueous NaClsolution (100 ml), dried over Na₂SO₄, filtered and concentrated on arotary evaporator. The crude product was purified by means of columnchromatography (SiO₂, n-hexane/ethyl acetate 4:1; R_(f)=0.35). 17.72 g(73%) of a yellowish liquid was obtained.

¹H-NMR (CDCl₃, 400 MHz): δ=2.35 (s, 3H; CH₃), 3.15 (t, 1H; J=4.4 Hz;OH), 4.60 (d, 2H; J=4.4 Hz; CH), 6.74 (s, 1H; ═CH).

¹³C-NMR (CDCl₃, 100.6 MHz): δ=14.7 (CH₃), 59.4 (CH₂OH), 108.2 (═C),127.7 (═CH), 134.2 (═C), 140.8 (═C).

3^(rd) Stage:(4-Bromo-5-methyl-thiophen-2-ylmethoxy)-tert.-butyldimethylsilane

Imidazole (6.36 g; 93.5 mmol) was added to a solution of4-bromo-2-hydroxymethyl-5-methylthiophene (17.60 g; 85.0 mmol) andtert.-butyldimethylchlorosilane (14.09 g; 93.5 mmol) in anhydrousdichloromethane (100 ml) and the suspension was stirred at RT. After 2h, the reaction mixture was washed with diluted hydrochloric acid (IN;100 ml), dried over Na₂SO₄, filtered and concentrated on a rotaryevaporator. The crude product was dissolved in n-hexane (50 ml) andfiltered over silica gel (SiO₂, n-hexane). The filtrate was concentratedon a rotary evaporator and the residue was dried under fine vacuum.25.84 g (95%) of a colourless oil was obtained.

¹H-NMR (CDCl₃, 400 MHz): δ=0.11 (s, 6H; Si—CH₃), 0.93 (s, 9H; C—CH₃),2.36 (s, 3H; CH₃), 4.75 (s, 2H; O—CH₂), 6.70 (s, 1H; ═CH).

¹³C-NMR (CDCl₃, 100.6 MHz): δ=−5.3 (CH₃), 14.7 (CH₃), 18.3 (C), 25.8(CH₃), 60.5 (CH₂), 107.9 (═C), 126.0 (═CH), 133.1 (═C), 142.2 (═C).

²⁹Si-NMR (CDCl₃, 79.5 MHz): δ=21.6.

4^(th) Stage:3,3,4,4,5,5-Hexafluoro-1,2-bis((tert.-butyldimethvl-silyl)oxymethyl-2-methyl-3-thienyl)cyclopent-1-ene

A solution of(4-bromo-5-methyl-thiophen-2-ylmethoxy)-tert.-butyldimethylsilane (25.74g; 80.0 mmol) in anhydrous tetrahydrofuran (100 ml) under argon wascooled to −75° C. n-Butyllithium (2.5M in n-hexane; 32.6 ml; 82.0 mmol)was added dropwise and the yellow solution was stirred for 2 h at −75°C. Octafluorocyclopentene (8.48 g; 40.0 mmol) was added and the reactionmixture was stirred overnight in a thawing cold bath. After 20 h, water(100 ml) and ethyl acetate (300 ml) were added to the reaction mixtureand the phases were separated. The organic phase was washed with water(2×100 ml). The combined aqueous phases were re-extracted with ethylacetate (100 ml). The combined organic phases were washed with saturatedaqueous NaCl solution (100 ml), dried over Na₂SO₄, filtered andconcentrated on a rotary evaporator. The crude product was purified bymeans of column chromatography (SiO₂, n-hexane/ethyl acetate 20:1;R_(f)=0.63). 19.46 g (74%) of a brownish liquid was obtained.

¹H-NMR (CDCl₃, 400 MHz): δ=0.11 (s, 12H; Si—CH₃), 0.94 (s, 18H; C—CH₃),1.88 (s, 6H; CH₃), 4.80 (s, 4H; O—CH₂), 6.87 (s, 2H; ═CH).

¹³C-NMR (CDCl₃, 100.6 MHz): δ=−5.3 (CH₃), 14.4 (CH₃), 18.3 (C), 25.8(CH₃), 60.6 (CH₂), 111.1 (m; CF₂), 113.7 (m; CF₂), 116.2 (m; CF₂), 118.8(m; CF₂), 123.1 (═CH), 124.3 (═C), 135.8 (m, ═C—CF), 141.1 (═C), 143.5(═C).

¹⁹F-NMR (CDCl₃, 376.5 MHz): δ=−131.9 (2F), −110.0 (4F).

²⁹Si-NMR (CDCl₃, 79.5 MHz): δ=21.7.

5^(th) Stage:3,3,4,4,5,5-hexafluoro-1,2-bis(5-hydroxymethyl-2-methyl-3-thienyl)cyclopent-1-ene

Tetrabutyl ammonium fluoride (75% in water; 25.00 g; 66.9 mmol) wasadded dropwise to a solution of3,3,4,4,5,5-hexafluoro-1,2-bis[(tert.-butyldimethylsilyl)oxymethyl-2-methyl-3-thienyl)cyclopent-1-ene(19.20 g; 29.2 mmol) in tetrahydrofuran (100 ml). The reaction solutionwas stirred for 4 h at RT, then saturated aqueous NH₄Cl solution (100ml) and ethyl acetate (100 ml) were added and the phases were separated.The organic phase was washed with water (2×100 ml) and saturated aqueousNaCl solution (100 ml), dried over Na₂SO₄, filtered and concentrated ona rotary evaporator. The crude product was purified by means of columnchromatography (SiO₂, n-hexane/ethyl acetate 1:1; R_(f)=0.38). 4.35 g(10.2 mmol; 35%) of a colourless solid was obtained.

1H-NMR (CDCl₃, 400 MHz): δ=1.87 (s, 6H; CH₃), 4.26 (s, 2H; OH), 4.68 (s,4H; O—CH₂), 6.91 (s, 2H; ═CH).

¹³C-NMR (CDCl₃, 100.6 MHz): δ=14.3 (CH₃), 59.1 (CH₂), 110.8 (m; CF₂),113.4 (m; CF₂), 116.0 (m; CF₂), 118.5 (m; CF₂), 124.0 (═CH), 135.7 (m,═C—CF₂), 141.4 (═C), 143.1 (═C).

¹⁹F-NMR (CDCl₃, 376.5 MHz): δ=−131.9 (2F), −110.0 (4F).

6^(h) Stage:3,3,4,4,5,5-Hexafluoro-1,2-bis(5-methacryloyloxymethyl-2-methyl-3-thienyl)cyclopent-1-ene

A solution of methacrylic anhydride (3.21 g; 20.8 mmol) indichloromethane (20 ml) was added dropwise to a solution of3,3,4,4,5,5-hexafluoro-1,2-bis(5-hydroxymethyl-2-methyl-3-thienyl)cyclopent-1-ene(4.25 g; 9.9 mmol), triethylamine (2.21 g; 21.8 mmol) andN,N-dimethylaminopyridine (0.24 g; 2.0 mmol) in anhydrousdichloromethane (80 ml) at −5° C. The reaction mixture was stirred for 3h at −5° C. and then at ambient temperature. After 20 h, the solutionwas washed with water (3×100 ml) and saturated aqueous NaCl solution(100 ml), dried over Na₂SO₄, filtered and concentrated on a rotaryevaporator. The crude product was purified by means of columnchromatography (SiO₂, n-hexane/ethyl acetate 9:1; R_(f)=0.40). 4.68 g(8.3 mmol; 84%) of a colourless solid was obtained (m.p.: 58° C.).

¹H-NMR (CDCl₃, 400 MHz): δ=1.86 (s, 6H; CH₃), 1.95 (m, 6H; CH₃), 5.24(s, 4H; O—CH₂), 5.61 (m, 2H; ═CH), 6.14 (m, 2H; ═CH); 7.05 (s, 2H; ═CH).

¹³C-NMR (CDCl₃, 100.6 MHz): δ=14.2 (CH₃), 18.2 (CH₃), 60.5 (CH₂), 110.9(m; CF₂), 113.4 (m; CF₂), 116.0 (m; CF₂), 118.5 (m; CF₂), 124.3 (═C),126.3 (═CH₂), 127.5 (═CH), 135.8 (═C), 136.1 (m, ═C—CF₂), 136.4 (═C),143.2 (═C), 166.9 (C═O).

¹⁹F-NMR (CDCl₃, 376.5 MHz): δ=−131.9 (2F), −110.2 (4F). IR (neat): 2958(w), 1716 (s), 1637 (m), 1561 (w), 1441 (m), 1404 (w), 1380 (w), 1337(m), 1316 (m), 1273 (s), 1191 (m), 1136 (vs), 1109 (vs), 1046 (s), 1011(m), 984 (vs), 942 (s), 898 (m), 856 (m), 813 (s), 740 (m), 709 (w), 656(m), 635 (w) cm⁻¹.

Analysis calculated for C₂₅H₂₂F₆O₄S₂: C, 53.19; H, 3.93; S, 11.36.

Found: C, 53.20; H, 3.76; S, 11.10.

1. A radically polymerizable dental material, characterized in that itcontains at least one compound of Formula (I),

in which the variables have the following meanings: X, Y independentlyof each other are O, S, Se, CH₂, N—R¹¹ or CR¹²R¹³, wherein R¹¹ to R¹³independently of each other in each case are a branched or unbranchedC₁-C₁₆ alkyl radical, aryl or —CH₂-aryl; R¹, R² independently of eachother in each case are a branched or unbranched C₁-C₃ alkyl radical,wherein these alkyl radicals can be substituted by one or more fluorineatoms; R⁵; R⁶ independently of each other in each case are halogen, CN,COOH, COOR⁴, CH₂OH, CO—NH₂, CH₂—NH₂, wherein R⁴ is a C₁-C₆ alkylradical, aryl or alkyl-aryl, or R⁵ and R⁶ together form a —(CH₂)_(n)—,—C(═O)—O—C(═O)— or —C(═O)—NR³—C(═O)— group, wherein n is 3 or 4 and R³is H, a C₁-C₆ alkyl radical, aryl or alkyl-aryl and wherein in the—(CH₂)_(n)— group one or more or all of H atoms can be replaced by F;R⁷⁻¹⁰ independently of each other in each case are H, a C₁-C₁₂ alkylradical, which can be interrupted by O, —O—C(═O)—NH— or phenylene, or anaromatic C₆-C₁₀ hydrocarbon radical, wherein these radicals in each casecan carry a radically polymerizable group, or R⁷ and R⁸ and/or R⁹ andR¹⁰, together with the C atoms to which they are bonded, form a benzenering, which can be unsubstituted or which can carry 1 to 4 substituents,which are selected from halogen, CN, —CO-aryl, —CO—CH₂-aryl, —CO—O-aryl,—CO—O—CH₂-aryl, branched or unbranched C₁-C₁₆ alkyl radicals, —O-alkyl,—CO-alkyl and —CO—O-alkyl, wherein alkyl in each case stands for abranched or unbranched C₁-C₁₆ alkyl radical and wherein all alkylradicals can be substituted by one or more fluorine atoms.
 2. The dentalmaterial according to claim 1, in which the variables of Formula I havethe following meanings: X, Y independently of each other are O, S orN—R¹¹, wherein R¹¹ is a branched or unbranched C₁-C₆ alkyl radical; R¹,R² independently of each other in each case are a C₁-C₂ alkyl radical,wherein these alkyl radicals can be substituted by one or more fluorineatoms; R⁵; R⁶ together form a —(CH₂)_(n)— group, wherein n is 3 or 4 andwherein in the —(CH₂)_(n)— group one, or more, or all H atoms can bereplaced by F; R⁷⁻¹⁰ independently of each other in each case are H or aC₁-C₆ alkyl radical, which can be interrupted by O or —O—C(═O)—NH—,wherein one or more H atoms can be substituted by fluorine atoms andwherein 1 or 2 of the radicals R⁷⁻¹⁰ carry a radically polymerizablegroup, or a (meth)acrylate group.
 3. The dental material according toclaim 2, in which the variables of Formula I have the followingmeanings: X, Y in each case are S; R¹, R² in each case are methyl; R⁵;R⁶ together form a —(CH₂)_(n)— group, wherein n is 3 and wherein all Hatoms are replaced by F; R⁷, R⁹ independently of each other in each caseare H or a C₁-C₃ alkyl radical; R⁸, R¹⁰ independently of each other ineach case are a C₁-C₆ alkyl radical, which can be interrupted by O or—O—C(═O)—NH— and which carries a terminal radically polymerizable group,or a methacrylate group.
 4. The dental material according to claim 1,wherein R⁷ and R⁸ stand for a group with Formula Ia and R⁹ and R¹⁰ standfor a group with Formula Ib

and, together with Formula I, form Formula (II):

in which R^(a), R^(b), R^(c), R^(d) and R^(a′), R^(b′), R^(c′), R^(d′)independently of each other in each case have the following meanings: H,halogen, CN, a branched or unbranched C₁-C₁₆ alkyl radical, —O-alkyl,—CO-alkyl, —CO—O-alkyl, wherein alkyl in each case stands for a branchedor unbranched C₁-C₁₆ alkyl radical and wherein all alkyl radicals can besubstituted by one or more fluorine atoms, —CO-aryl, —CO—CH₂-aryl,—CO—O-aryl, —CO—O—CH₂-aryl; wherein the radicals R^(a), R^(b), R^(c),R^(d) or R^(a′), R^(b′), R^(c′), R^(d′) can be linked to each other,forming one or more non-aromatic or aromatic rings or annulated aromaticring systems, which have 2 to 5 rings, wherein the rings or ring systemscan be substituted or unsubstituted.
 5. The dental material according toclaim 4, wherein the variables have the following meanings, which can bechosen independently of each other: X, Y independently of each other ineach case are 0, N—R¹¹, wherein R¹¹ is a branched or unbranched C₁-C₁₆alkyl radical, N-aryl or N—CH₂-aryl; R¹, R² independently of each otherin each case are a branched or unbranched C₁-C₆ alkyl radical, whereinthe alkyl radicals can be substituted by one or more fluorine atoms; R⁵,R⁶ independently of each other in each case are halogen, CN, COOH,COOR⁴, wherein R⁴ is H, alkyl, aryl, alkyl-aryl, CH₂OH, CO—NH₂ orCH₂—NH₂, or R⁵ and R⁶ together form a —C(═O)—O—C(═O)—, —C(═O)—NR³—C(═O)—or —(CH₂)_(n)— group, wherein n is 3 or 4 and one or more or all H atomscan be replaced by F and wherein R³ is H or a C₁-C₆ alkyl radical, arylor alkyl-aryl; R^(a), R^(b), R^(c), R^(d) and R^(a′), R^(b′), R^(c′),R^(d′) independently of each other are H, halogen, a branched orunbranched C₁-C₁₆ alkyl radical, —O-alkyl, —CO—O-alkyl, wherein alkyl ineach case stands for a branched or unbranched C₁-C₁₆ alkyl radical andwherein all alkyl radicals can be substituted by one or more fluorineatoms, —CO-aryl, —CO—CH₂-aryl, —CO—O-aryl, —CO—O—CH₂-aryl; wherein theradicals R^(a), R^(b), R^(c), R^(d) or the radicals R^(a′), R^(b′),R^(c′), R^(d′) can be linked to each other, forming one or more aromaticrings, or an annulated aromatic ring system with 2 to 3 rings, whereinthe rings or ring systems can be substituted or are unsubstituted. 6.The dental material according to claim 4, wherein the variables have thefollowing meanings: X, Y independently of each other in each case are O,N—R¹¹, wherein R¹¹ is a branched or unbranched C₁-C₁₆ alkyl radical; R¹,R² independently of each other in each case are a methyl group, whichcan be substituted by one or more fluorine atoms; R⁵, R⁶ independentlyof each other in each case are COOH, CH₂OH, CO—NH₂, CH₂—NH₂ or R⁵ and R⁶together form a —C(═O)—O—C(═O)— or —(CH₂)_(n)— group, wherein n is 3 or4 and one or more, or all H atoms can be replaced by F, or R⁵ and R⁶together form a —(CF₂)₃— group; R^(a), R^(b), R^(c), R^(d) and R^(a′),R^(b′), R^(c′), R^(d′) independently of each other are H, halogen, abranched or unbranched C₁-C₁₆ alkyl radical, —O-alkyl, wherein alkylstands for a branched or unbranched C₁-C₁₆ alkyl radical and wherein allalkyl radicals can be substituted by one or more fluorine atoms, whereinthe radicals R^(a), R^(b), R^(c), R^(d) or the radicals R^(a′), R^(b′),R^(c′), R^(d′) can be linked to each other, forming one or more aromaticrings, which are not substituted.
 7. The dental material according toclaim 1 which additionally contains at least one radically polymerizablemonomer, or a mono- or polyfunctional (meth)acrylic acid derivative, andat least one initiator or photoinitiator for the radical polymerization.8. The dental material according to claim 1, which additionally containsat least one particulate filler.
 9. The Dental material according toclaim 1, which contains a) 0.0001 to 5.0 wt.-% of at least one compoundof general formula I, b) 0.01 to 10 wt.-% of at least one initiator, andoptionally c) 0 to 80 wt.-% other monomer(s), and optionally d) 0 to 80wt.-% filler(s), and optionally e) 0 to 70 wt.-% solvent, in each caserelative to the total mass of the dental material.
 10. The dentalmaterial according to claim 9 for use as dental cement or dental fillingcomposite, which contains a) 0.0001 to 5.0 wt.-% of at least onecompound of general formula I, b) 0.01 to 10 wt.-% of at least oneinitiator, c) 0 to 50 wt.-% other monomer(s), d) 10 to 80 wt.-%filler(s).
 11. The dental material according to claim 9 for use asdental adhesive or coating material, which contains a) 0.0001 to 5.0wt.-% of at least one compound of general formula I, b) 0.01 to 10 wt.-%of at least one initiator, c) 0 to 80 wt.-% other monomer(s), d) 0 to 20wt.-% filler(s), e) 0 to 70 wt.-% solvents.
 12. The dental materialaccording to claim 1 for intraoral use to restore damaged teeth.
 13. Thedental material according to claim 12 for therapeutic use as dentalcement, dental filling composite, dental adhesive or veneering material.14. A photochromic dental material comprising a compound of Formula I,as defined in claim
 1. 15. The dental material according to claim 3,which additionally comprises at least one mono- or polyfunctionalmethacrylate and at least one photoinitiator.
 16. The dental materialaccording to claim 9, which contains a) 0.001 to 3.0 wt.-% of at leastone compound of general formula I, b) 0.1 to 3.0 wt.-% of at least oneinitiator, and optionally c) 0 to 60 wt.-% other monomer(s), andoptionally d) 0 to 80 wt.-% filler(s), and optionally e) 0 to 70 wt.-%solvent, in each case relative to the total mass of the dental material.17. The dental material according to claim 9, which contains a) 0.01 to1.0 wt.-% of at least one compound of general formula I, b) 0.1 to 3.0wt.-% of at least one initiator, and optionally c) 5 to 50 wt.-% othermonomer(s), and optionally d) 0 to 80 wt.-% filler(s), and optionally e)0 to 70 wt.-% solvent, in each case relative to the total mass of thedental material.
 10. 18. The dental material according to claim 10 foruse as dental cement or dental filling composite, which contains a) 00.001 to 3.0 wt.-% of at least one compound of general formula I, b) 0.1to 3.0 wt.-% of at least one initiator, c) 0 to 40 wt.-% othermonomer(s), d) 20 to 80 wt.-% filler(s).
 19. The dental materialaccording to claim 10 for use as dental cement or dental fillingcomposite, which contains a) 0.01 to 1.0 wt.-% of at least one compoundof general formula I, b) 0.1 to 3.0 wt.-% of at least one initiator, c)5 to 40 wt.-% other monomer(s), d) 30 to 80 wt.-% filler(s).
 20. Thedental material according to claim 11 for use as dental adhesive orcoating material, which contains a) 0.001 to 3.0 wt.-% of at least onecompound of general formula I, b) 0.1 to 3.0 wt.-% of at least oneinitiator, c) 5 to 60 wt.-% other monomer(s), d) 0 to 20 wt.-%filler(s), e) 0 to 60 wt.-% solvents, wherein the solvents comprisewater and/or ethanol.
 21. The dental material according to claim 11 foruse as dental adhesive or coating material, which contains a) 0.01 to1.0 wt.-% of at least one compound of general formula I, b) 0.1 to 3.0wt.-% of at least one initiator, c) 5 to 50 wt.-% other monomer(s), d) 0to 20 wt.-% filler(s), e) 0 to 50 wt.-% solvents, wherein the solventscomprise water and/or ethanol.